Levels of serotonin were measured in microdissected, individual brain nuclei in ovariectomized rats after treatment with ovarian hormones. Regions sampled included nuclei in the forebrain, rostral and medial hypothalamus, and midbrain tegmentum. Estradiol benzoate decreased levels of lutenizing hormone in plasma but did not after serotonin levels in any region. Progesterone alone elevated serotonin content in the nucleus tractus diagonalis and ventral tegmental area. The combined estrogen plus progesterone regime produced a surge in plasma luteinizing hormone and also markedly elevated serotonin in the median eminence.